Pipette for dispensing successive volumes of liquid

ABSTRACT

The invention relates to a pipette for dispensing successive volumes of liquid by repeated action on a pusher (22). A drive lever (26) is provided to drive a pushrod (15) and a piston rod (5) downwards only. The lever (26) extends transversely to the longitudinal axis (X) of the thrust rod (15) and it includes a hole (27) through which the pushrod (15) passes with a small amount of clearance. The pusher (22) can act on one end of the lever (26) to cause it to tilt relative to the pushrod (15) and then to drive the pushrod downwards by wedging, together with the piston rod (5), and to expel a predetermined volume of liquid. The lever (26) is mounted to rotate about a tilt axis (Y) that intersects the axis of the hole (27) perpendicularly substantially halfway along the hole.

FIELD OF THE INVENTION

The present invention relates to a pipette for dispensing successivevolumes of liquid by repeated action on a pusher.

BACKGROUND OF THE INVENTION

French patent application No. 2 432 161 describes a pipette comprising ahousing of oblong shape incorporating a mechanism for adjusting thevolume to be dispensed, and a syringe for fixing removably to the bottomend of said housing. A piston suitable for moving in sealed mannerinside the syringe body is mounted at the bottom end of a pushrodcapable of sliding longitudinally through the housing. In order todispense a predetermined volume of liquid, the pushrod is drivendownwards by action on a pusher via a drive lever that is capable ofmoving over a predetermined stroke between a fixed bottom abutment andan adjustable top abutment. The lever comprises a first portion piercedby a hole enabling the pushrod to pass therethrough with smallclearance, and a second portion extending transversely to the axis ofsaid hole. The pusher is disposed to exert a pressure force in thevicinity of the free end of the second portion of the lever, and by awedging effect to drive the thrust rod. The lever is also subjected toupwards resilient biasing so as to ensure that it returns automaticallyto its rest position against the top abutment when the pusher isreleased. That type of pipette has given satisfaction. However, in thelong run, wear on the various component parts of the pipette gives riseto reduced accuracy.

SUMMARY OF THE INVENTION

An object of the present invention is to propose a novel pipettepresenting greater accuracy than known pipettes, even after prolongeduse.

The pipette of the invention is of the type comprising:

a housing of oblong shape, incorporating a mechanism for adjusting thevolume of liquid to be dispensed;

a syringe for mounting removably to one end of the housing, comprising asyringe body provided with a fluid-passing orifice and a piston rodmounted to move inside said syringe body;

a thrust rod mounted to slide longitudinally in the housing, and to beremovably connected at one end to said piston rod;

a drive lever for driving the thrust rod and the piston rod solelytowards said liquid-passing orifice, the lever extending transversely tothe longitudinal axis of the thrust rod and being provided with a holethrough which the thrust rod passes with a small amount of clearance,the lever also being mounted with the ability to move inside the housingover a stroke that is adjustable in the longitudinal direction of thethrust rod between a rest position defined by an adjustable abutment ofthe adjustment mechanism and an end-of-stroke position defined by afixed abutment, and being returned towards said rest position byresilient return means; and

said pushbutton being mounted to slide longitudinally inside thehousing, being provided at one end extending outside the housing with apushbutton and being capable of acting via its other end on the lever totilt it obliquely relative to the thrust rod and then to drive thethrust rod by wedging towards the fluid-passing orifice together withthe piston rod so as to expel a predetermined volume of liquid asdefined by the stroke of the lever between said rest position and saidend of stroke position, return movement of the lever towards its restposition after each action on the pushbutton taking place under drivefrom said resilient return means while the piston rod retains itsposition within the body of the syringe.

In characteristic manner, the lever is mounted to rotate about an axisthat intersects the axis of said hole perpendicularly and substantiallyhalfway along the hole.

Thus, however much the clearance for the rod in the hole through thelever may increase due to wear, the lever continues to tilt about thesame geometrical axis of rotation until the thrust rod is wedged in thehole; if there is considerable wear, then the lever tilts further, butunlike known pipettes, that has no effect on the stroke of the thrustrod. In known pipettes, the lever is not guided during its tiltingmovement and said movement can be accompanied by a small amount ofsliding over the thrust rod, with the result that the thrust rod is notdriven over the full distance defined by the difference between thefixed and adjustable abutments, with the result that the accuracy of thepipette is reduced.

Advantageously, the lever is disposed on the thrust rod between a bottomsleeve and a top sleeve slidably mounted on the thrust rod, and thelever has two surfaces of revolution centered on said tilt axis, saidsurfaces being disposed on either side of said hole and being in contactwith the top and bottom sleeves. The bottom sleeve comes into abutmentagainst said fixed abutment when the lever reaches its end-of-strokeposition.

Advantageously, said surfaces of revolution are formed by balls crimpedin holes through the lever.

Advantageously, said pusher includes a sheath slidably mounted insidethe housing and within which the pushrod is free to slide. It isprovided at one end with a contact piece having a sharp edge designed topress on the lever to tilt it about said tilt axis and then to move italong the longitudinal direction of the thrust rod.

Advantageously, the adjustment mechanism includes an adjustment screwfor rotating in order to adjust the stroke of the lever and the volumedispensed each time the pusher is actuated. This adjustment screw hasthe sheath and the pushrod passing through the inside thereof, andmeshing with a volume-indicator mechanism. The adjustment screw has anend edge that defines said variable abutment, and the top sleeve has anend edge suitable for coming into point contact therewith.

Advantageously, the pushbutton includes a first portion slidably mountedover a second portion, with a first return spring being interposedtherebetween. A second return spring of stiffness constant smaller thanthat of the first return spring is resiliently interposed between thehousing and said second portion to return the pusher resiliently awayfrom said liquid-passing orifice. The stiffness constant of saidresilient return means urging the lever towards its rest position issmaller than the stiffness constants of the first and second returnsprings.

Advantageously, the syringe and the piston rod are removably fixed tothe housing of the pipette by means of a clamp in the form of a partwhose top portion is secured to the bottom end of the thrust rod, whosemiddle portion includes a recess suitable for receiving the free top endof the piston rod, and whose bottom portion includes resilient branchesenabling the piston rod to be inserted therein and to be held thereby,and the bottom end of the housing includes means suitable for removablysecuring the body of the syringe while leaving the clamp and the pistonrod free to slide

Advantageously, the middle portion of the clamp includes a recessdefined by the free top faces of the clamps, by a transverse surface,and by a longitudinal surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention appear onreading the following detailed description of non-limiting embodimentsof the invention, and on examining the accompanying drawings, in which:

FIGS. 1 and 1a are longitudinal section views through two differentembodiments of a pipette of the invention;

FIG. 2 is a fragmentary longitudinal section through the pipette shownin FIG. 1, but on a larger scale;

FIG. 3 is a perspective view of the drive lever through which the thrustrod passes;

FIGS. 4 to 7 show successive positions of the drive lever while thepipette is in use;

FIGS. 8 to 10 show successive positions of the pushbutton fitted to thepusher of the pipette while the pipette is in use;

FIG. 11 is a perspective view of a clamp for fixing the piston tail; and

FIGS. 12 and 13 show how the syringe-piston assembly is put into placeby pivoting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the description below, the terms "bottom" and "top" are used todescribe the pipette when in its generally vertical in-use position.

The pipette of the invention as shown in FIG. 1 comprises a housing 1 ofplastics material, of oblong shape and suitable for being grasped by auser. A syringe 2 is removably mounted at the longitudinally bottom endof the housing 1 to take and to dispense successive volumes of liquid.The syringe 2 has a body 3 that is generally circularly cylindricalabout an axis X, and that is extended downwards by a frustoconicalend-piece 4 having an open end 4' for passing liquid. A piston rod 5 ofplastics material is mounted to slide in sealed manner inside the body3, with sealing being provided by an annular lip 6 situated in thebottom portion of the piston rod 5. Beyond the lip 6, the piston rod hasa conical bottom end 7 shaped to engage in the frustoconical end-piece 4so as to expel liquid contained therein as completely as possible whenthe piston rod 5 reaches its bottom end-of-stroke position. The top end8 of the body 3 has an outwardly projecting flange which is retained inan annular groove 9 in the housing 1 by swelling or by any otherequivalent disposition enabling the syringe and piston assembly to bemounted and dismounted easily on the open bottom end of the housing 1.

A particularly advantageous embodiment of a fixing part enabling theassembly comprising the syringe 2 and the piston rod 5 to be mounted anddismounted, both simultaneously and rapidly, is described below ingreater detail with reference to FIGS. 1a, 11, 12 and 13.

The annular groove 9 is formed in the radially inside surface of areceptacle 10 which is generally circularly cylindrical about the axisX, which is formed in the bottom portion of the housing 1, and in whichthe piston rod 5 of the syringe 2 can rise while liquid is being taken.The wall 11 of the housing 1 that defines the receptacle 10 is generallycircularly cylindrical about the axis X, and it is open parallel to theaxis X along a fraction of its length so as to form a longitudinal slot12 for passing a drive finger 13 for driving the piston rod 5 in amanner described below.

The top end of the piston rod 5 has a coupling portion 14 releasablyconnected to the bottom end of a thrust rod 15 mounted to slidelongitudinally inside the housing 1 along the axis X.

The top portion of the thrust rod 15 is slidably mounted in the housing1 and is circularly cylindrical about the axis X, while its bottom endflares to form a circularly cylindrical receptacle 16 about the axis Xfor the purpose of receiving the coupling portion 14 of the piston rod5. This coupling portion 14 is ringed, presenting a sequence of threefriction disks 17 of the same diameter and uniformly spaced apart alonga core 18 of smaller diameter than the disks 17. The coupling portion 14is retained inside the receptacle 16 by friction. The drive finger 13 isfixed to the wall 19 of the thrust rod 15 where it defines thereceptacle 16, and it passes through the slot 12.

In the embodiment shown in FIG. 1a, the drive finger 13a is raisedtowards the top portion of the housing 1 so as to facilitate access tothe drive finger 13 by a user manipulating the pipette with one handonly. As can be seen in FIG. 1a, that naturally assumes the finger 13aextends inside the housing in the form of a kind of strip 13b connectingit to the part for controlling and fixing the tail of the piston. Thebottom portion of the strip 13b is terminated by a tongue of brightcolor that becomes visible outside the housing at the end of the pipettestroke, i.e. when the body of the syringe 2 is about to be completelyemptied.

The top portion of the housing 1 contains a mechanism 20 for adjustingthe volume to be dispensed, which mechanism is described in greaterdetail below. It also contains an assembly 21 coupled to the adjustmentmechanism 20 for indicating said volume. A pusher 22 comprising a metalsheath 40 fitted at its top end with a pushbutton 23 is slidably mountedinside the housing 1 on the axis X so as to enable the user to dispensesuccessive volumes of liquid by repeated downward presses on thepushbutton 23.

Each time the user exerts pressure on the pushbutton 23, the sheath 40moves down and, via a one-way transmission 24, drives the thrust rod 15and the piston rod 5 downwards, i.e. towards the liquid-passing orifice4' so as to dispense liquid, whereas the upward return movement of thesheath 40 takes place solely under drive from resilient return meansconstituted solely by a helical spring 25 and independently of thethrust rod 15 and the piston rod 5, each of which remains in the sameposition inside the housing 1 and inside the body 3 of the syringe 2under the effect of the friction exerted by the lip 6 on the body 3 ofthe syringe 2.

In the embodiment of FIG. 1a, the resilient return means are implementedin the form of a helical spring 25a which does not work in compressionlike the spring 25, but which works in traction.

The one-way transmission 24 that is now described more particularly withreference to FIG. 2 includes a lever 26 extending generally transverselyto the axis X, and having a central circular hole 27 through which thethrust rod 15 passes with a small amount of clearance. This smallclearance allows the lever 26 to move between a rest position where theaxis of the central hole 27 is substantially parallel to the axis X(thus enabling the lever 26 to move freely over the thrust rod 15) and aposition that is downwardly oblique relative to the axis X (to drive thethrust rod 15 by a wedging effect).

The drive lever 26 is disposed on the thrust rod 15 between a top sleeve28 and a bottom sleeve 29, both of which are slidably mounted thereon. Ahelical spring 30 working in compression is placed between the bottomsleeve 29 and a ring 31 that serves as a fixed abutment, and that isretained axially against an inside shoulder 32 of the housing 1. Thebottom face of the ring 31 closes the receptacle 10, while its sidefacing away from the housing has a central portion 33 which iscircularly cylindrical about the axis X, which has a bore for passingthe thrust rod 15, and it also has a wall 34 that is circularlycylindrical about the axis X and that co-operates with the centralportion 33 to form an upwardly open annular groove 35. The wall 34 hasits radially outer surface in contact with the housing 1.

The bottom end 36 of the helical spring 30 bears axially against thebottom of the annular groove 35, while its top end 37 bears against ashoulder 38 on the bottom sleeve 29. The sheath 40 of the pusher 22 iscircularly cylindrical about the axis X, and the metal thrust rod 15 isfreely slidable therein.

The bottom end of the sheath 40 is extended by a contact part 41designed to bear against the lever 26 so as to tilt it and then, afterwedging against the thrust rod 15, so as to displace the lever 26 alongthe axis X together with the thrust rod 15. The contact part 41 has afirst portion 44 connected at one end to the sheath 40 and extendingradially outwards perpendicularly to the axis X over a distance that isgreater than the outside radius of the sheath 40, so as to connect to asecond portion 46 that extends obliquely downwards away from the axis X,forming an angle of about 45° therewith. The bottom end of the secondportion 46 has a sharp edge 47 designed to exert pressure in thevicinity of the radially outermost free end 48 of the lever 26(outermost in the plane of the section of FIGS. 1 and 2). The top sleeve28 is generally circularly cylindrical about the axis X; it has a borerunning from its top end edge 39 and along more than half of its lengthfor the purpose of receiving the bottom end of the sheath 40. The wall42 of the top sleeve 28 defining the bore designed to receive the sheath40 has an opening in its periphery that forms a longitudinal slot 43through which the contact part 41 passes.

The assembly constituted by the lever 26 and the top and bottom sleeves28 and 29 moves inside a guide part 49 that is generally tubular aboutthe axis X, that is downwardly open and closed at the top, apart fromtapping 63 centered on the axis X and suitable for receiving a hollowadjustment screw 62 within which the sheath 40 and the thrust rod 15 canslide. The guide part 49 has a shouldered bottom end 50 engaging in theannular groove 35 of the ring 31, in contact with the radially innersurface of the wall 34 until the shoulder comes into abutment againstthe top edge of the wall 34. The top end edge 51 of the guide part 49comes to bear against the volume-indicating assembly 21. The guide partis open along its entire length so as to form a guide slot 52 for theportion 46 of the contact part 41 and for the end 48 of the lever 26,thereby ensuring that they are held by the guide part 49 insubstantially vertical mutual relationship; this ensures that when thesheath 40 moves downwards, the edge 47 always comes to bear against theend 48 of the lever 26.

In FIG. 3, the lever 26 and the thrust rod 15 are shown in perspective.The lever is mainly constituted by a plate extending perpendicularly tothe axis of the central hole 27, having a first portion 53 with thecentral hole 27 passing through the center thereof to allow the thrustrod 15 to pass through with a small amount of clearance, and a secondportion 54 that extends the first portion 53 laterally so as to act asan abutment for the contact part 41. Two holes 55 and 56, situated oneither side of the central hole 27, pass through the central portion 53,with each of them serving to receive a respective ball 55 or 57 that isheld in the plate by crimping. The plane P₂ containing the axis of thecentral hole 27 and the axes of the holes 55 and 56 containing the balls57 and 58 extends perpendicularly to a mid-plane P₁ of the lever 26,likewise containing the axis of the central hole 27 and extending in adirection towards the second portion 54 of the lever 26. In the exampledescribed, the section of the second portion 54 in a plane parallel tothe plane P₂ is rectangular in shape.

The balls 57 and 58 are axially interposed between the bottom sleeve 29and the top sleeve 28, and they constitute guide means for guidingtilting of the lever 26 when the contact part 41 extending the sheath 40downwards bears against the end 48 of the second portion 54. Thistilting takes place about an axis of rotation Y that intersects the axisX perpendicularly, substantially halfway along the central hole 27. Inaccordance with the invention, more or less marked clearance due to wearin the passage provided for the thrust rod 15 by the central hole 27 ofthe lever 26 gives rise to the lever 26 tilting to a greater or lesserextent about its axis of rotation Y, until the thrust rod 15 wedgesagainst the edges of the central hole 27. The axis of rotation Y of thelever 26 remains stationary throughout the tilting of the lever.

The top end wall 39 of the top sleeve 28 has an upwardly convexprojection 60 whose apex defines point contact with the bottom end edge61 of the adjustment screw 62. The sheath 40 passes through the insideof the screw, as mentioned above, and the outside of the screw isthreaded so that its bottom portion engages the tapping 63 in the guidepart 49. The assembly constituted by the adjustment screw 62 and theguide part 49 constitutes the above-mentioned mechanism 20 for adjustingthe volume to be dispensed by serving to vary the downward stroke of thelever 26, and thus that of the thrust rod 15 and of the piston rod 5,each time the pusher 22 is actuated, as is described in greater detailbelow. The top end of the adjustment screw 62 has a collar 64 that isaccessible to the user from outside the housing 1 via openings formed inthe housing 1 and not shown. The radially outer surface of the collar 64is advantageously knurled so as to facilitate manual rotation of theadjustment screw 62 by the user. A brake may also be applied to theadjustment screw 62 so as to exert a friction force relative to thehousing 1 and prevent untimely loss, in use, of the adjustment of thevolume as selected by the user. The collar 64 of the adjustment screw 62rotates inside a part 65 that is generally circularly cylindrical aboutthe axis X and that is provided with openings designed to come intocoincidence with the openings in the housing 1 so as to give access tothe collar 64 from the outside. A closure washer 66 is applied to thetop end edge of the part 65. The bottom portion of the part 65 ispierced to receive a nut 67 through which the adjustment screw 62passes.

The indicator assembly 21 is known per se and is not described indetail. In the example shown, it is of the type comprising three drumsfor indicating the volume to be dispensed, which drums are juxtaposedand mesh in one another and with the adjustment screw 62 in conventionalmanner. The volume to be dispensed is indicated to the user by the drumsvia a transparent window 68.

The operation of the pipette is now described more particularly withreference to FIGS. 4 to 7.

FIG. 4 shows the drive lever 26 in its rest position. The top sleeve 28comes into axial abutment via the projection 60 against the bottom endedge of the adjustment screw 62. The top sleeve 28 is held in thisposition by the force from the helical spring 30 which is transmitted tothe top sleeve 28 by means of the balls 57 and 58 and the bottom sleeve29. The lever 26 extends substantially perpendicularly to the axis X,and there is no wedging of the thrust rod 15 in the central hole 27. Thesheath is urged resiliently upwards by the spring 25. It is held insidethe adjustment screw 62 by the contact part 41 coming into abutmentagainst the bottom end of the screw.

FIG. 5 shows the sheath 40 of the pusher 22 at the end of a first strokereferred to as a "comfort" stroke, extending over a distance B from therest position of the sheath 50 until the edge 47 comes into contact withthe leer 26. When the user continues to press on the pushbutton 23, i.e.beyond the end of the comfort stroke, the lever 26 is caused to tiltabout the axis Y until the thrust rod 15 wedges within the central hole27. As mentioned above, the tilting movement takes place about the axisY without sliding over the thrust rod 15. Therefore, the bottom sleeve29 does not move during tilting of the lever 26. When the user pressesfurther on the pushbutton 23, the sheath 40 acts via the contact part 41to drive the lever 26 downwards, and the movement of the lever isaccompanied by movement of the thrust rod 15 because the thrust rod iswedged in the central hole 27. The downward movement of the lever 26 istransmitted to the bottom sleeve 29 by the balls 57 and 58, and thesleeve moves downwards against the helical spring 30 until the bottomend edge 70 of the bottom sleeve 29 comes into abutment against theplane top end edge 71 of the central portion 33 of the ring 31, whichedge extends generally perpendicularly to the axis X. The thrust rod 15has moved down through a distance A corresponding to the initialdistance between the bottom end edge 70 of the bottom sleeve 29 and thetop end edge 71 of the central portion 33 of the ring 31. As it movesdownwards, the thrust rod 15 entrains the piston rod 5, and apredetermined volume of liquid is expelled through the orifice 4'. Whenthe user releases pressure on the pushbutton 23, the lever 26 moves backup on its own, independently of the thrust rod 15, because there is nowedging between the thrust rod and the hole 27 when the contact part 41is not exerting pressure on the end 48. The helical spring 30 returnsthe lever 26 towards its rest position, which corresponds to the topsleeve 28 coming into abutment against the bottom end edge 61 of theadjustment screw 62.

By acting on the adjustment screw 62, the user can alter the initialdistance between the bottom end edge 70 of the bottom sleeve 29 and thetop end edge 71 of the central portion 33 of the ring 31, and thus canalter the distance A and the volume which will be dispensed on the nextoccasion that pressure is exerted on the pushbutton 23.

As shown more particularly in FIGS. 8 to 10, the pushbutton 23preferably comprises two portions that are assembled togethertelescopically. More precisely, the pushbutton 23 includes a bottomportion 81 having a bore 82 for engaging on the top end edge of thesheath 40 until the sheath comes into abutment against the end of thebore 82. Externally, the bore 82 is surrounded in a downward directionby a cylindrical wall 83 that co-operates with the sheath 40 to form adownwardly open annular groove 84 for receiving the top end of thespring 25. The spring operates in compression and is interposedresiliently between the bottom of the groove 84 and the closure washer66 having the sheath 40 passing through the center thereof. Thepushbutton 23 also includes a top portion 80 which is mounted to slidealong the axis X over the bottom portion 81. The bottom portion isprovided with retaining tongues 86 at its top end, each of which has anoutwardly projecting tooth suitable for coming into abutment against ashoulder 87 formed on the radially inner surface of the top portion 80so as to retain it axially. A helical spring 88 operating in compressionis mounted inside the top portion 80 to be interposed resilientlybetween the top front face 89 of the bottom portion 81 and the bottom 90of the top portion 80, and to urge the top portion resiliently upwardsuntil the teeth of the tongues 86 come into abutment against theshoulder 87.

Before using the pipette, the syringe 2 is filled with a liquid to bedispensed by immersing the frustoconical end-piece 4 in the liquid to betaken, and then manually moving the piston rod 5 up into the receptacle10 by means of the drive finger 13. The thrust rod 15 moves up freelyinside the sheath 40 of the pusher 22, since the lever 26 is in its restposition.

To dispense a predetermined volume of liquid, as selected by rotatingthe collar 64 and as indicated by the drums of the display assembly 21,the user presses on the top portion 80 of the pushbutton 23. The thrustforce is transmitted via the spring 88 to the bottom portion 81, andthus to the sheath 40, which sheath moves downwards until the bottomsleeve 29 comes into abutment against the ring 31. The spring 25 forreturning the sheath 40 to its rest position is then compressed. If theuser continue to press on the pushbutton 23, the top portion 80 coversthe bottom portion 81 as shown in FIG. 10. The user only becomes awarethat the spring 88 is compressed after the bottom sleeve 89 has reachedthe ring 31. This ensures that the user pushes the pushbutton 23 over adistance that is sufficient to ensure that the pushrod 15 is drivendownwards over the entire stroke A. When the user begins to release thepushbutton 23, the bottom sleeve 29 remains in abutment for the lengthof time taken by the spring 88 to relax. This limits jerking on thepushrod 15 and ensures that the entire volume of liquid to be dispensedis indeed delivered. The stiffness constant of the spring 88 is greaterthan those of the springs 25 and 30. The stiffness constant of thespring 25 is greater than that of the spring 30.

Finally, the invention makes it possible to obtain constant accuracy forthe pipette, in spite of wear affecting the central hole 27 and thethrust rod 15. It is not limited to the embodiment described. Inparticular, it is possible to modify the fixing of the syringe 2relative to the housing 1 and the means for providing a connectionbetween the piston rod 5 and the thrust rod 15.

FIG. 11 is a perspective view of a particularly advantageous embodimentof fixing means for fixing the assembly comprising the syringe 3 and thepiston rod 5 to the bottom end of the housing 1 of the pipette. Itcomprises a clamp 100 specially designed to enable the bottom portion ofthe pipette to be installed and replaced quickly. The clamp 100 isconstituted by a solid part disposed at the bottom end of the housing 1of the pipette in the position shown by the sections of FIG. 1a and ofFIGS. 12 and 13. It comprises an open portion provided with tworesilient branches 101 that open to the outside to facilitate guidanceand to enable the end of the piston rod 5 to be inserted. The bottomfaces of the two resilient branches 101 are connected together by acylindrical surface 102 in which the tail of the piston rod 5 is held.

In practice, it is advantageous to use a cylindrical surface whoseinside diameter is slightly greater than that of the outside surface ofthe tail of the piston rod, by about 1/10th of a millimeter. As shown inFIG. 11, the resilient branches 101 are located in the bottom portion ofthe clamp 100 whose middle portion includes a recess designed to enablethe piston tail to be-mounted and dismounted. This recess is defined bythree generally plane surfaces which are essential for providingmounting and retention of the piston tail described above in greaterdetail. Adjacent the opening between the branches 101, there is ashoulder surface 103 which is constituted by a small plane surfaceextending on the longitudinal axis of the pipette and serving as ahooking and bearing surface for the top friction disk 17 on the tail ofthe piston rod 5. The hooking function of the shoulder surface 103 isillustrated by the position of the piston syringe assembly shown in FIG.12. This is the position for the piston syringe assembly for insertion,which takes place laterally with the assembly inclined relative to thebottom of the pipette housing. The piston assembly is put finally intoplace merely by pivoting said assembly so as to bring it into alignmentwith the pipette. For this purpose, the clamp 100 includes a transversesurface 104 that is not accurately perpendicular to the axis X of thepipette, but that slopes by a few degrees relative thereto, thusestablishing a set-back that allows the top disk 17 of the piston rod 5to pivot. This surface is thus therefore not properly speaking anabutment or a thrust surface.

Finally, on the side opposite to the opening of the branches 101, themiddle portion of the clamp 100 includes a longitudinal surface 105 forbearing transversely against the tail of the piston rod. It can be seenin FIG. 13 that, after the piston and syringe assembly has tilted, thevarious friction disks 17 come into contact with said surface 105 alongone of their generator lines. Naturally, to improve retention, thepipette also includes a metal clamp 106 of appropriate resilience thatlocks the bottom portion of the pipette in its proper working position,as shown in FIG. 13. The essential function of the clamp 106 is to holdthe syringe 2 pressed against the bottom support collar 107 of thepipette housing, while the piston rod 5 is free to move together withthe clamp 100 inside the receptacle 10 of the pipette.

The top portion 108 of the clamp 100 includes means for securing it tothe top end of the thrust rod 15 to control longitudinal displacementsof the assembly comprising the clamp 100 and the piston 5 within saidreceptacle 10 of the pipette.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by Letters Patent of theUnited States is:
 1. A pipette for dispensing successive volumes ofliquid by repeated action on a pushbutton (22), the pipette comprising:ahousing (1) of oblong shape, incorporating an adjustment mechanism foradjusting the volume of liquid to be dispensed; a syringe (2) formounting removably to one end of the housing (1), comprising a syringebody (3) provided with a fluid-passing orifice (4') and a piston rod (5)mounted to move inside said syringe body (3); a thrust rod (15) mountedto slide longitudinally in the housing, and to be removably connected atone end to said piston rod (5); a drive lever (26) for driving thethrust rod (15) and the piston rod (5) solely towards saidliquid-passing orifice (4'), the lever (26) extending transversely to alongitudinal axis (X) of the thrust rod (15) and being provided with ahole (27) through which the thrust rod (15) passes with an amount ofclearance, the lever (26) also being mounted with the ability to moveinside the housing over a stroke that is adjustable in a direction alongthe longitudinal axis (X) of the thrust rod (15) between a rest positiondefined by an adjustable abutment of the adjustment mechanism and anend-of-stroke position defined by a fixed abutment (31), and beingreturned towards said rest position by resilient return means (30); anda pusher (22) being mounted to slide longitudinally inside the housing,being provided at one end extending outside the housing with apushbutton (23) and being capable of acting via its other end on thelever (26) to tilt it obliquely relative to the thrust rod (15) and thenby wedging the thrust rod (15) to drive the thrust rod (15) towards thefluid-passing orifice (4') together with the piston rod (5) so as toexpel a predetermined volume of liquid as defined by the stroke of thelever (26) between said rest position and said end-of-stroke position,return movement of the lever (26) towards its rest position after eachaction on the pusher (22) taking place under drive from said resilientreturn means (30) while the piston rod (5) retains its position withinthe body (3) of the syringe (2), the pipette being characterized in thatthe lever (26), mounted to rotate about a tilt axis (Y) that intersectsthe axis of said hole (27) perpendicularly and substantially halfwayalong the hole, is disposed on the thrust rod (15) between a bottomsleeve (29) and a top sleeve (28) slidably mounted on the thrust rod(15), and wherein said lever has two surfaces of revolution centered onsaid tilt axis (Y), said surfaces being disposed on either side of saidhole (27) and being in contact with the top and bottom sleeves (28, 29),the bottom sleeve (29) coming into abutment against said fixed abutment(31) when the lever (26) reaches its end-of-stroke position.
 2. Apipette according to claim 1, characterized in that said surfaces ofrevolution are formed by balls (57, 58) crimped in holes (55, 56)through the lever (26).
 3. A pipette according to claim 2, characterizedin that said pusher (22) includes a sheath (40) slidably mounted insidethe housing and within which the thrust rod (15) is free to slide, andwherein the sheath (40) is provided at one end with a contact piece (41)having a sharp edge (47) designed to press on the lever (36) to tilt thelever (26) about said tilt axis (Y) and then to move the lever (26)along the longitudinal axis (X) of the thrust rod (15).
 4. A pipetteaccording to claim 3, characterized in that the adjustment mechanismincludes an adjustment screw (62) for rotating in order to adjust thestroke of the liver (26) and the volume dispensed each time the pusher(22) is actuated, said adjustment screw (62) having the sheath (40) andthe thrust rod (15) passing through the inside thereof, and meshing witha volume-indicator mechanism (31), the adjustment screw (62) having anend edge (61) that defines said variable abutment, and wherein the topsleeve (28) has an end edge suitable for coming into point contact withthe end edge (61) of the adjustment screw (62) defining said variableabutment.
 5. A pipette according to claim 1, characterized in that thepushbutton (23) includes a first portion (80) slidably mounted over asecond portion (81), with a first return spring (88) being interposedtherebetween, a second return spring of stiffness constant smaller thanthat of the first return spring being resiliently interposed between thehousing (1) and said second portion (81) to return the pusher (22)resiliently away from said liquid-passing orifice (4'), the stiffnessconstant of said resilient return means (30) urging the lever (26)towards its rest position being smaller than the stiffness constants ofthe first and second return springs (88).
 6. A pipette according toclaim 1, characterized in that the syringe (2) and the piston rod (5)are removably fixed to the housing (1) of the pipette by means of aclamp (100) in the form of a part whose top portion (108) is secured toa bottom end of the thrust rod (15), whose middle portion includes arecess suitable for receiving a free top end of the piston rod (5), andwhose bottom portion includes resilient branches (101) enabling thepiston rod (5) to be inserted therein and to be held thereby, andwherein a bottom end of the housing includes means (106, 107) suitablefor removably securing the body (3) of the syringe (2) while leaving theclamp (100) and the piston rod (5) free to slide.
 7. A pipette accordingto claim 6, characterized in that the middle portion of the clamp (100)includes a recess defined by free top faces of the branches 101, by atransverse surface (104) and by a longitudinal surface (105).